EXTINCT HORSES. 5 



the lateral toes being represented only by the so-called ^^ splint- 

 bones '' (tig. 7). 



Remains of Horses indistinguishable from some of the various 

 forms of the existing species occur in the superficial deposits of 

 Europe and Asia, in company with those of the Mammoth. At a 

 somewhat earlier epoch (Pliocene) occur Horses, such as E. stenonis 

 of Europe and E. sivalensis of India, in which the head is 

 relatively larger, the feet are somewhat smaller, the splint-bones 

 more developed, and the skull shows traces of a depression in 

 front of the eye. The American Pliohippus is smaller, with 

 shorter cheek-teeth. Still earlier (Miocene) is found in America 

 a Horse known as Merychippus or Protohippus in which the 

 splint-bones are fully developed and terminate inferiorly in small, 

 although perfect, toes. In the early Pliocene Hipparion, or 

 three-toed Horse, the lateral toes are still larger, while the crowns 

 of the cheek-teeth are lower, and the skull is shorter and shows a 

 large depression in front of the eye. In this animal the crowns of 

 the cheek-teeth are still tall and have their hollows filled with 

 cement (fig. 6, E), and there must consequently be some unknown 

 forms connecting it with the Miocene Anchit her turn, in which the 

 crowns of these teeth are quite short, and have their hollows free 

 from cement. Hipparion is generally regarded as ofi" the direct 

 ancestral line. 



This type is common to Europe, Asia, and North America ; but 

 Mr. J. W. Gidley, in the Bulletin of the American Museum, has 

 come to the conclusion that the New World Hipparions are 

 generically distinct, and proposes that they should be known as 

 Neohipparion. They differ from the Old World forms by certain 

 details of tooth-structure, as well as by their more slender limbs, 

 in which it seems that the lateral toes are relatively smaller. 

 Finally, they are of Miocene, instead of Pliocene, age. 



Nearly allied to Anc hit her iu7n is the Oligocene ^enns Mesa hippuSj 

 the species of which are smaller than the typical representative of 

 the former. In these animals the socket of the eye is open behind, 

 the gap between the canine and cheek-teeth is comparatively 

 short, the lateral toes are functional, and there is even a suggestion 

 of a fourth toe in the fore-foot (fig. 2, B). This digit is fully 

 developed in the fore-foot (fig. 2, A) of Hyracotherium^ a Lower 



